1. Field of the Invention
This invention relates to a method of reducing a wave front aberration of an optical wave traversing a projection system of a lithographic apparatus and a computer program product comprising program code to control a lithographic apparatus to perform a device manufacturing method.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g. a silicon wafer). The lithographic apparatus comprises an illumination system to illuminate the mask and a projection system (also referred to as a projection lens) to transfer the pattern, via imaging, onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Known lithographic apparatus include so-called steppers or step-and-repeat apparatus, and so-called scanners or step-and-scan apparatus. In a stepper, each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and the wafer is moved by a predetermined amount to a next position for a next exposure. In a scanner, each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction, and next the wafer is moved to a next position for a next exposure.
In the practice of optical lithography for the manufacturing of IC devices, there is a continuing desire to enhance the lithographic projection apparatus performance in terms of throughput, i.e., the number of wafers that can be exposed per unit of time. The throughput is directly proportional to the power of the radiation provided by a radiation source which is coupled to the illumination system.
With increasing power also any residual absorption of radiation traversing the projection lens by material of optical elements or coatings on optical elements is becoming more critical. The beam of radiation propagating through the projection lens causes a local, generally non-uniform heating of optical elements. Such a heating may cause thermal deformations of the projection lens elements and hence, an optical wave aberration error.
A wave aberration is usually considered as being composed of a sum of weighted, basic wave-aberration components, the set of basic wave aberration components being a set of spatial phase-distributions described by a corresponding set of normalized, orthogonal polynomials expressed in projection lens pupil coordinates in a plane perpendicular to an optical axis of the projection lens.
Conventional methods to compensate or partially compensate a wave aberration change include applying small displacements or rotations from nominal position and nominal orientation of projection lens optical elements (lenses, groups of lenses ), or applying small deformations of lens element shapes. Thereto, a projection lens is equipped with a limited number of lens manipulators. Lens manipulators are connected to a controller for calculating and applying manipulator settings and manipulator-setting changes. Wave aberration errors which are rotationally symmetric with respect to the optical axis of the projection lens or lower order wave aberration errors (for example, described by polynomials of second order in the pupil coordinates) can be reduced using the manipulator settings. However, compensation results obtained with the conventional techniques in the presence of higher-order wave-aberration errors are not satisfactory. Uncorrectable higher-order wave-aberration errors remain present in the image forming radiation beam near or at the substrate. Such a residual wave aberration is a source of errors for a latent pattern image as well as for processed, printed patterns. For example feature size errors, and pattern-asymmetry errors may be present.